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Almost 30 million Americans were considered to be current illicit drug users in 2014, meaning that they had abused drugs within the month leading up to that year’s National Survey on Drug Use and Health (NSDUH). Drugs interact with the brain and body to alter moods, emotions, and behaviors by changing brain chemistry and a person’s perceptions, and by impacting how individuals interact with the world around them.

Mind-altering drugs may slow down or speed up the central nervous system and autonomic functions necessary for living, such as blood pressure, respiration, heart rate, and body temperature. Levels of some of the brain’s chemical messengers, or neurotransmitters, are also impacted by drug abuse, including:

Dopamine: This neurotransmitter regulates moods, enhances pleasure, and is involved with movement, reward and reinforcing behaviors, motivation, and attention.

Serotonin: This neurotransmitter is responsible for stabilizing moods and regulating emotions.

Gamma-aminobutyric acid (GABA): GABA acts as a natural tranquilizer, mitigating the stress response and lowering anxiety levels as well as slowing down functions of the central nervous system.

Norepinephrine: Similar to adrenaline, norepinephrine is often called the “stress hormone,” as it speeds up the central nervous system in response to the “fight-or-flight” response. It also homes focus and attention while increasing energy levels

Regions of the brain are disrupted by drug abuse, as the National Institute on Drug Abuse (NIDA) reports that the brain stem, limbic system, and cerebral cortex are all affected. The brain stem controls life-sustaining functions, including sleeping, breathing, and heart rate, while the limbic system holds the brain’s reward circuitry and helps to control emotions and the ability to feel happiness. The cerebral cortex is considered the “thinking center” of the brain, managing problem-solving, planning, and decision-making abilities as well as helping people to process information provided by their senses.

The more often drugs are used, the more they will impact brain chemicals and circuitry, which can lead to drug dependence and withdrawal symptoms when the drugs process out of the body. Drug cravings, dependence, and withdrawal symptoms, coupled with a loss of control over use, are signs of addiction.

The American Society of Addiction Medicine (ASAM) defines addiction as a disease affecting brain chemistry and circuitry, which then leads to compulsive drug-seeking and using behaviors. In 2014, nearly 22 million Americans battled addiction, NSDUH reports.

Marijuana

Marijuana is the most regularly used illicit drug in the United States, and its use is especially common among adolescents and young adults, NIDA reports. The psychoactive chemical in marijuana, delta-9-tetrahydrocannabinol (TCH), interacts and binds with cannabinoid receptors in the brain, producing a mellowing and relaxing effect.

Regions of the brain with high concentrations of cannabinoid receptors are heavily impacted. One such part of the brain, the hippocampus, manages short-term memory, meaning that marijuana use can impede recollection of recent events. Additional regions of the brain that are impacted include the cerebellum and basal ganglia, which help to control coordination and involuntary muscle movements respectively. When someone abuses marijuana, impaired motor skills, mood alterations, distorted time and sensory perception, decreased memory, and trouble thinking clearly and solving problems are all common short-term side effects. Marijuana also interferes with levels of dopamine in the brain, causing the euphoric “high” that users document.

Marijuana also has several long-term side effects on the brain, which are especially prevalent in individuals who use the drug before the brain is fully developed. NIDA warns that marijuana use in adolescence, and continued on into adulthood, may result in a loss of IQ points that are not recoverable even with abstinence. Also, initiating marijuana use before turning age 18 raises the risk for addiction as an adult. As many as 30 percent of those who use marijuana will suffer from addiction to the drug, and the risk is increased 4-7 times when use begins before the age of 18. As a person ages, neurons in the hippocampus are naturally lost, and marijuana use may speed up this process, leading to memory problems. Impaired coordination, learning issues, and sleep problems can result from long-term marijuana use and its impact on the brain.

NIDA also reports on the possible link between marijuana use and the onset of psychosis and psychiatric disorders like schizophrenia in those who are genetically vulnerable. Breathing and respiration problems, chronic cough, and bronchitis are additional possible consequences of chronic marijuana smoking. Marijuana use can also disrupt heart rhythm and normal cardiac functions.

Synthetic Cannabinoids

Synthetic marijuana, also known as Spice or K2, acts on the brain in a similar fashion to the marijuana but to a greater extent. These drugs have been manufactured to be more potent and may then be more active in the brain than the naturally occurring form. Synthetic cannabinoids are generally full agonists at cannabinoid receptor type-1, or CB1, receptors in the brain whereas pot is only a partial agonist. These drugs may be 100 or more times potent than the THC found in natural marijuana, Forbes warns.

The journal World Psychiatry reports that there are over 200 forms of synthetic cannabinoids marketed today. Each may have a slightly different chemical or molecular structure and can have unpredictable effects on the brain and body.

Heroin and Prescription Opioids

Heroin and prescription opioid drugs like OxyContin (oxycodone), Vicodin (acetaminophen/hydrocodone), fentanyl, methadone, and Dilaudid (hydromorphone) bind to opioid receptors in the brain and trigger the release of dopamine. In a sense, these drugs hijack the limbic system in the brain, inducing a powerful high that individuals are often keen to recreate, leading to reinforcing behaviors.

Opioid drugs are considered highly addictive, as ASAM publishes that almost a quarter of heroin users will suffer from addiction to opioids. Over 2.5 million Americans battled opioid addiction in 2015.

Heroin is considered the fastest-acting opioid, taking effect nearly immediately and making it extremely addictive, the Drug Enforcement Administration (DEA) warns.

When someone takes an opioid drug repeatedly, they can develop a tolerance to it as the body gets used to its interaction in the brain. Individuals may then take more of the drug to feel the desired effects. The brain will then stop functioning as it did before introduction of the opioid, causing levels of dopamine to drop when the drug wears off.

Dependence on opioids can form rather quickly. Physical withdrawal symptoms may resemble the flu, and emotional withdrawal symptoms include depression, anxiety, and insomnia.

Opioid drugs also disrupt the natural production of norepinephrine and act as central nervous system depressants. Opioids block pain sensations, induce drowsiness, reduce body temperature, and slow heart rate, blood pressure, and respiration functions. Opioid overdose is an all too common consequence of opioid abuse, which can often result in severe respiratory depression that can be fatal. The Centers for Disease Control and Prevention (CDC) reports that 60 percent of all overdose deaths in 2015 involved an opioid drug, and 91 people in the United States die from an opioid overdose each day.

Long-term, chronic heroin use may also result in the deterioration of some of the brain’s white matter, which can negatively impact the way a person responds to stress, regulates emotions, and makes decisions, NIDA publishes. Lung complications and infections of the lining of the heart are additional long-term concerns surrounding perpetuated opioid drug abuse.

Cocaine, Methamphetamine, and Other Stimulants

Cocaine, methamphetamine (meth), and prescription amphetamines, such as those used to treat attention deficit hyperactivity disorder (ADHD) like Adderall (amphetamine/dextroamphetamine) and Ritalin (methylphenidate) are classified as stimulant drugs. This means that they speed up the central nervous system, increasing heart rate, body temperature, and blood pressure while increasing energy levels, focus, attention, alertness, and wakefulness. They also suppress appetite.

Cocaine and meth, especially, produce an intense high as they rapidly flood the brain with dopamine. The high is generally fairly short-lived, however, and cocaine is often abused in a binge pattern to try and extend the euphoria. The “crash” that comes after a stimulant high can be significant, leaving a person feeling extremely fatigued, hungry, irritable, mentally confused, and depressed, which is followed by intense cravings. Stimulant drugs are extremely addictive due to the way they impact dopamine levels and affect the limbic reward system.

Regular cocaine abuse can lead to paranoia and negatively impact functions of the central nervous system, causing cardiac arrhythmias, sudden cardiac arrest, ischemic heart conditions, a respiratory syndrome unique to snorting to cocaine, hypertension, convulsions, stroke, and death, the DEA warns. Meth has similar effects on the brain and central nervous system, and individuals who abuse it regularly may suffer from hallucinations, anxiety, and confusion as well. Meth also significantly damages the dopamine system in the brain, which can cause problems with memory and learning, movement, and emotional regulation issues.

NIDA warns that even long-term abstinence may not reverse all of the negative brain changes incurred by meth abuse. Such abuse may also increase the risk for the onset of Parkinson’s disease, a nerve disorder impacting movement.

Benzodiazepines

Benzodiazepine drugs are prescription sedatives and tranquilizers, such as Valium (diazepam), Ativan (lorazepam), Xanax (alprazolam), and Klonopin (clonazepam). These drugs are prescribed to treat anxiety, to relieve muscle tension, and as sleep aids. They serve to increase levels of GABA in the brain and slow functions of the central nervous system. They are commonly misused, however, and can have a euphoric effect when taken in large doses.

Regular use of them, even when taken as directed through a valid prescription, can cause tolerance to develop, requiring escalating dosages. Higher doses, or using the drugs in a manner other than intended (e.g., crushing tablets to snort, smoke, or inject the powder; chewing tablets; or taking too many at once) can lead to extreme confusion, impaired reflexes and coordination, coma, and death. Mixing a benzodiazepine with alcohol, opioids, or other central nervous system depressants greatly increases the risk for a life-threatening overdose.

The prescribing information published by the U.S. Food and Drug Administration (FDA) for Xanax warns that it is not to be taken as a long-term solution since the drug can lead to physical and psychological dependence when taken in doses higher than 4 mg/day for more than 12 weeks. After a dependence on a benzodiazepine has formed, the brain can experience a rebound effect when the drug leaves the bloodstream. The central nervous system that was dampened by the benzo can go into overdrive, and the brain may be slow to produce GABA on its own, which can result in elevated anxiety, depression, trouble sleeping, tremors, suicidal tendencies, sweating, hypertension, irregular heart rate, muscle tension and aches, nausea and vomiting, and even potentially life-threatening seizures.

These drugs have such an impact on the central nervous system and brain function that they should not be discontinued suddenly once a dependence has formed. Instead, they are generally tapered off gradually, with the dosage being slowly lowered over a safe period of time. Medical supervision is required for safe withdrawal.

Ecstasy

Ecstasy, also known as Molly or by its chemical name, MDMA, is a popular club and psychoactive drug. It binds to serotonin transporters in the brain and has both stimulant and hallucinogenic properties. Within about an hour after ecstasy enters the bloodstream, it stimulates the activity of serotonin, norepinephrine, and dopamine, NIDA explains.

Ecstasy is also commonly combined with alcohol or other drugs, or “cut” with toxic substances, which can have potentially hazardous consequences. Molly, often heralded as the “pure” form of ecstasy, may contain any number of adulterants or chemicals that can have toxic effects, NBC News warns.

If a person takes additional doses of MDMA while the drug is still in the system, it can interfere with the metabolism, which can make the cardiovascular and toxic side effects worse, NIDA warns. While the majority of the side effects of MDMA wear off in a few hours, confusion and anxiety can last up to a week after taking ecstasy.

MDMA interferes with the way the brain processes information and stores memories, and with long-term use, these cognitive issues can become more pronounced. Anxiety, irritability, sleep difficulties, depression, aggression, impulsivity, loss of appetite, and decreased interest in sex may be side effects of regular ecstasy use. Ecstasy may also be psychologically addictive, leading to withdrawal symptoms when the drug isn’t taken.

LSD, PCP, Ketamine, and Hallucinogens

A class of drugs that leads to distortions of reality and perceptions, hallucinogens are typically broken down into two main categories: classic hallucinogens (LSD, peyote, psilocybin, DMT, Ayahuasca) and dissociative drugs (PCP, salvia, DXM, ketamine), per NIDA. It is not certain exactly how these drugs work in the brain; however, it is largely understood that they interrupt normal communication between neurotransmitters.

Dissociative drugs are believed to disrupt the action of glutamate, a brain chemical that is involved with memories, cognition, emotions, and how people perceive pain. PCP interacts with dopamine as well, while salvia activates the kappa opioid receptor present on nerve cells, per NIDA. Dissociative drugs can make people feel separate from themselves, their environment, and reality. This can result in impaired motor functions, auditory and visual distortions, memory loss, anxiety, numbness, and body tremors.

NIDA further reports that classic hallucinogens are thought to interact mostly with serotonin and the prefrontal cortex of the brain. Alterations of moods, distortions of reality and sensory perceptions, and seeing, hearing, or feeling things that are not there are common side effects of drug-induced psychosis, or a “trip.” Some people may feel euphoric and have what they consider to be a spiritual awakening while others may suffer from panic, paranoia, anxiety, and despair, which are side effects of a “bad trip.” Hallucinogenic drugs can be unpredictable and affect each user differently.

Hallucinogenic and dissociative drugs can also interact with the central nervous system, causing irregularities in body temperature, heart rate, respiration, and blood pressure. Sedation, amnesia, muscle contractions and seizures, aggression and violence, psychotic symptoms resembling schizophrenia, and immobility may also occur. In general, hallucinogens are not considered to be addictive; however, there may be residual effects of their abuse.

A percentage of individuals (likely around 5 percent) may experience flashbacks, or a reoccurrence of symptoms, randomly and without warning months or even years after using a hallucinogenic drug, the journal Therapeutic Advances in Psychopharmacology publishes. Some may even suffer from a serious disorder called hallucinogen persisting perception disorder, or HPPD, which interferes with daily life functioning in the form of ongoing visual disturbances and hallucinations, or persistent psychosis, a series of mental problems that continue after drug use is stopped. Abusing a hallucinogenic drug, even one time, can have lasting and damaging effects on the brain and body.